Question

What are the anode and cathode reactions in a lead-acid storage battery? What happens when the battery is recharged?

Short answer

Anode reaction: \( Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^- \). Cathode reaction: \( PbO_2(s) + 4H^+(aq) + SO_4^{2-}(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l) \). Recharging reverses these reactions.

Step-by-step solution

Identify Electrodes in Lead-Acid Battery

In a lead-acid battery, the electrodes are made of lead and lead dioxide. The lead serves as the anode and the lead dioxide serves as the cathode.

Write Down the Discharging Reactions

During discharge, the anode undergoes oxidation (loses electrons) and the cathode undergoes reduction (gains electrons). The anode reaction is: \( Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^- \). The cathode reaction is: \( PbO_2(s) + 4H^+(aq) + SO_4^{2-}(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l) \).

Describe the Recharging Process

When the battery is recharged, an external electrical energy source forces the reactions to proceed in the reverse direction. The reactions during recharging are the reverse of the discharging reactions. The lead sulfate forms back into lead and lead dioxide, restoring the battery's ability to deliver electric power.

Key concepts

Electrochemical Cells

Electrochemical cells are the fundamental building blocks of batteries, transforming chemical energy into electrical energy through a redox reaction. A lead-acid battery, the type commonly found in vehicles, is a prime example of a galvanic cell, which is a particular type of electrochemical cell that releases energy spontaneously. In essence, it consists of two electrodes (the anode and the cathode) each made of different materials to facilitate electron flow.

Inside the lead-acid battery, the anode is created from lead (Pb), while the cathode is comprised of lead dioxide (PbO₂). These electrodes are immersed in an electrolyte solution, often sulfuric acid (H₂SO₄), that allows ions to move between the electrodes. This ion movement, along with the flow of electrons in the external circuit, is what generates an electric current. It's important to note that each half-cell in the battery executes its part of the reaction separately, contributing to the complete circuit when connected.

Oxidation and Reduction

Oxidation and reduction are the twin halves of a redox reaction, essential to the functioning of batteries, including the lead-acid type. Oxidation involves the loss of electrons, while reduction is the gain of electrons.

Diving into the chemistry of a discharging lead-acid battery: the anode reaction involves the lead (Pb) being oxidized by losing electrons to become lead sulfate (PbSO₄). This process can be represented by the equation: \[ Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^- \].

Simultaneously, at the cathode, lead dioxide (PbO₂) gets reduced when it gains electrons, combining with hydrogen ions and sulfate ions to also form lead sulfate (PbSO₄) and water, as shown here: \[ PbO_2(s) + 4H^+(aq) + SO_4^{2-}(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l) \].

The movement of electrons from the anode to the cathode is what ultimately provides the electrical energy we harness from the battery.

Battery Recharging

Battery recharging is a restorative process that reverses the discharge reactions in batteries such as the lead-acid type. When a battery discharges, the stored chemical energy is depleted as it is converted into electrical energy, which is used to power electrical devices. The recharging process involves applying an external electric current to drive the chemical reactions in the opposite direction, restoring the battery's energy content.

During recharging, the lead sulfate (PbSO₄) that formed on both electrodes during discharge is converted back into lead (Pb) at the anode and lead dioxide (PbO₂) at the cathode, effectively regenerating the original materials of the battery's electrodes. In terms of chemical equations, the recharging process simply reverses the discharging reactions:

At the anode: \[ PbSO_4(s) + 2e^- \rightarrow Pb(s) + SO_4^{2-}(aq) \] and at the cathode: \[ PbSO_4(s) + 2H_2O(l) \rightarrow PbO_2(s) + 4H^+(aq) + SO_4^{2-}(aq) + 2e^- \].

With a renewed charge, the battery is once again capable of delivering electrical power, demonstrating the reversible nature of electrochemical cells.